How would you translate takemusu, and how would you explain takemusu aiki?

Yours sounds good. Saotome calls (or did when I was around him) "creative spontaneous harmony".

Jun has it:

Takemusu Aiki A "slogan" of the founder's meaning "infinitely generative martial art of aiki." Thus, a synonym for aikido. The scope of aikido is not limited only to the standard, named techniques one studies regularly in practice. Rather, these standard techniques serve as repositories of more fundamental principles (KIHON). Once one has internalized the KIHON, it is possible to generate a virtually infinite variety of new aikido techniques in accordance with novel conditions.